This deceptively simple question is often critical in silent
computing. Why? Because power consumption and thermal dissipation are essentially
one and the same. Silent computing is largely the art of intelligent power management,
and thermal management with minimal airflow.

It can be broken down into several different questions:

How much AC power does a computer and all its related peripherals (monitors,
speakers, and printers, for example) draw?

How much AC power does a computer draw apart from its peripherals?

How much DC power does each individual component in the computer system
draw?

How much power does a computer system draw from each DC voltage rail on
the power supply?

The first question is relevant when considering the electrical cost of a computer,
and is most likely to be asked by system integrators and IT departments with large numbers of systems where
even a small reduction in power per system may translate into a significant monetary
saving. It is useful to know how much each system as a whole costs to operate. However, this is not our focus.

The second question is of great interest to anyone interested in
assembling a quiet PC because it tells us the total heat produced
inside a computer case. Almost all power drawn by a computer ends up as
heat; AC power draw is the best measure of total heat
in the PC box. A system that draws 90W at full load is much easier to
keep cool (and do so quietly) than a system that draws 250W.

A Kill-a-Watt power meter used to measure AC power draw.

However, it's not just the total amount of heat that matters but where
it is being produced. In most systems there are two or three main sources of
heat: The CPU, the video card (if any) and the power supply. An effective means
of reducing heat is to replace any of these with a more efficient model, especially
if one of them is especially power hungry. It is helpful to know how much power
various components require, which is one reason for posing the third question
above.

Knowing the power requirements of each individual component is also useful
when "sizing" a power supply. Adding together the power required by each component
at full load can give a rough estimate of the maximum power that will be drawn
from the power supply. Just about every component inside a PC carries
some kind of rating for maximum power or current. But simply adding these numbers
together produces estimates for total system power demand that are always too
high, sometimes by as much as double. This is because no PC application stresses
all components simultaneously to maximum load.

This brings us to the last question, How much power does a computer system
draw from each DC voltage rail on the power supply?

Wecan answer this question and the question of how much power
is demanded by various components by measuring the actual current drawn on each
voltage line while the computer is in use. In fact, these are issues so often
discussed in the SPCR forums without any clear conclusions being reached that
we decided to measure the power distribution within half a dozen different PC
systems in an effort to shed a bit more light on the topic. The results should
bring some real empirical data to these discussions, and should help determine
what can  and what can't  be predicted about the power draw of a
system.